How to make a smooth color transition?

Question

Adobe Photoshop

How to make a smooth color transition like in this picture (from brown to light brown). The only tool that comes to mind is the gradient tool, but you can't draw complex shapes with it.

Answer 1

Layers, soft brushes, Gaussian blur and Smadge Tool (finger). Here, for example, I scribbled it in a minute.

Answer 2

Gradient map over black and white layer